This invention relates to a process for producing a component from an Al.sub.2 O.sub.3 /titanium aluminide composite material in which a shaped body is pressed from a starting mix containing titanium, in particular as an oxide, the shaped body is subjected to a heat treatment at a conversion temperature to produce a sacrificial body, the sacrificial body is filled with softened or liquid aluminum and/or an aluminum alloy under pressure at a filling temperature, and starting materials of the sacrificial body are reacted with the filled aluminum to form an Al.sub.2 O.sub.3 /titanium aluminide composite material as is known from DE 196 05 858 A1.
DE 196 05 858 A1 has disclosed a process for producing a component from an Al.sub.2 O.sub.3 /titanium aluminide composite material. The ceramic/metal composite material combines the properties of the ceramic and the metallic phases and has a high strength and a high fracture toughness. In the process on which the invention is based, a starting mix is formed containing, inter alia, titanium in the form of an oxide compound. The titanium oxide can be reduced by means of aluminum so as to simultaneously form aluminide and Al.sub.2 O.sub.3. One titanium oxide of the starting mix which may be mentioned is TiO.sub.2. A shaped body which is close to its final shape is pressed from the starting mix. The shaped body is converted, by means of a heat treatment at a conversion temperature, into a sacrificial body, which is then infiltrated with liquid aluminum. Before being filled with aluminum, the sacrificial body is sintered under pressure. After sintering, the temperature of the sacrificial body is set to a filling temperature which is above the melting temperature of aluminum and/or an aluminum alloy (referred to below, including the claims, as aluminum for simplification purposes). Furthermore, the filling temperature is below a reaction temperature at which a so-called SHS reaction takes place between the aluminum and at least one of the starting materials. An SHS reaction (self-propagating high-temperature synthesis) is a reaction which above its reaction temperature takes place very quickly, is highly exothermic and is at least almost uncontrollable. At the filling temperature, the sacrificial body is filled with aluminum under pressure. After filling, the filled sacrificial body is heated beyond the filling temperature to a transformation temperature which is above the filling temperature, at which an exchange reaction takes place between the aluminum and the constituents of the sacrificial body, so as to form an Al.sub.2 O.sub.3 /titanium aluminide composite material.
However, the sacrificial body, as is evident from the examples given in DE 196 05 858 A1, is only transformed into the Al.sub.2 O.sub.3 /titanium aluminide composite material in certain areas. Furthermore, it can also be seen from DE 196 05 858 A1 that a sacrificial body containing TiO.sub.2 can only be completely filled with aluminum in some instances. Furthermore, a sacrificial body of this nature can also only be completely provided with a titanium aluminide phase in exceptional cases, resulting in a high reject rate.
DE-P 19710671.4, which is not a prior publication, discloses a process for producing a component from a metal/ceramic composite material, in which a sacrificial body comprising ceramic precursor materials is filled with thermally softened metal--in particular aluminum--and/or with a metal alloy. The filling temperature is below a reaction temperature, at which reaction temperature an exchange reaction between a metal of the ceramic precursor material and a metal of the filling metal takes place. After the sacrificial body has been filled as completely as possible, the filled sacrificial body is heated to the transformation temperature or above, as a result of which the exchange reaction just mentioned then takes place. This exchange reaction produces a component made from the metal/ceramic composite material which comprises a ceramic phase and a metallic phase with an intermetallic bond between the metal of the ceramic and the metal of the filling metal. As a result of the sacrificial body being filled with a metal which has been softened by heating at below a reaction temperature at which an exchange reaction takes place between the filling metal and the material of the sacrificial body, the ceramic matrix is retained during filling and also during the subsequent exchange reaction between the introduced metal and the material of the sacrificial body. Ideally, the pores of the sacrificial body are completely filled, so that when the substances in question are used in stoichiometric amounts, the component has reacted fully all through and is free of cracks and channels. Preferably, the filling metal is aluminum and the metal of the ceramic is titanium, so that after the preferred exchange reaction the ceramic phase comprises TiB.sub.x and/or TiC.sub.y and/or TiCN and Al.sub.2 O.sub.3, and the intermetallic compound of the metallic phase is a high-temperature-resistant titanium aluminide, in particular TiAl. The material properties of this metal/ceramic composite material are good. Thus, for example, a metal/ceramic composite material which is produced using aluminum as filling metal and Ti as metal of the ceramic sacrificial body has a density of 3.4 g/cm.sup.3 ; this density is slightly higher than that of the MMCs (metal matrix composites) but is only 42% of the density of comparable cast iron. Particularly in the preferred embodiment, in which the high-temperature-resistant compound is in the form of the intermetallic compound TiAl, the use range of the component extends to at least 800.degree. C., significantly above the values for grey cast iron. The metal/ceramic composite material produced is used, in particular, to manufacture friction rings for the frictional surfaces of disc brakes. These friction rings are subsequently fixed by means of mechanical joining techniques, such as screws, etc., to the hub of the brake disc.
However, before the sacrificial body is filled with the metal or the alloy, the starting materials of the sacrificial body have to be heated, a first exchange reaction taking place between the precursor materials, in which reaction high-grade, expensive precursor materials form from the exchange materials. After filling with the metal, the ceramic phase and the metallic phase are formed from these expensive precursor materials and the metal, an exchange reaction again being used to form these phases, in this case between the precursor material and the filling metal.
A further process likewise describes the infiltration of a ceramic sacrificial body with aluminum (U.S. Pat. No. 4,988,645). In this process, the ceramic body is produced using an SHS reaction (SHS reaction: self-propagating high-temperature synthesis, meaning the ignition of a reactive mixture with the reaction propagating itself and providing the desired ceramic matrix as reaction products).
However, some components produced in this way have unacceptable levels of porosity, and consequently the reject rate is high. In particular, the filling with sacrificial bodies containing TiO.sub.2 as precursor material of the sacrificial body is very poor.
In general terms, all the above methods have a high energy requirement, which is attributable, inter alia, to the various thermal processes, such as sintering, first exchange reaction, filling and subsequent second exchange reaction at temperatures which are higher than the filling temperature. This energy requirement makes the processes expensive.
WO 84/02927 has disclosed a process for producing fiber-reinforced die-cast parts containing aluminum using the so-called squeeze-casting process. In the process, firstly a porous green body is pressed from a starting mix containing, inter alia, fibers, and this green body is then filled with aluminum. To stabilize the porous green body and to maintain the orientation of the fibers arranged in the green body, a binder is added to the starting mix and is removed by thermal means during filling of the green body. Due to the presence of the pores and the strength of the binder, the green body does not undergo any deformation, or at most only negligible deformation. In this case, there is no chemical reaction between the filling aluminum and the starting materials of the green body, and consequently the influence of such a reaction on the structure and form of the subsequent die-cast component is not known.